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Poster B64, Thursday, August 16, 3:05 – 4:50 pm, Room 2000AB

Restricted Diffusion Imaging (RDI) as a measure of axonal packing density in children and adults

Anthony Dick1, Dea Garic1;1Florida International University

Restricted diffusion imaging (RDI) is a novel diffusion-weighted neuroimaging metric that is proposed to measure cellular and axonal density (Yeh et al, 2016). This metric has been shown to be sensitive to tumors and inflammation within the rat brain, but it has never been tested to measure axonal density within the developing human brain. Our study aimed to use this in vivo imaging method to replicate the anterior-to-posterior distribution of axonal density in the corpus callosum, which is well-established in post-mortem tissue (Aboitiz et al, 1992). We hypothesized that, because it was designed to be sensitive to cellular density, relative to established diffusion metrics, this novel RDI metric would be specifically sensitive to the pattern of axonal packing density in the human corpus callosum. Our participants were 840 adults from the human connectome project (HCP) database (undisclosed exact ages, age range= 20-40 years) and 129 infants, children, and adolescents (M= 8.67 years) from the C-Mind database. In both samples, we were able to match the histological density patterns seen in post-mortem tissue. Specifically, contrast analyses showed a high degree of fit between the density patterns from the Aboitiz model and our adult sample, t(839) = 167.99 p < .001, and developing sample, t(126) = 227.4, p < .001. We also showed that the pattern was only apparent for RDI. The anterior-posterior distribution of other metrics, which included generalized fractional anisotropy (GFA), quantitative anisotropy (QA), fractional anisotropy (FA), radial diffusivity (RD), axial diffusivity (AD), and mean diffusivity (MD), did not match the pattern revealed in post-mortem tissue. These findings provide preliminary evidence in support of the RDI metric’s sensitivity to cellular density and could potentially have wide-reaching implications for future clinical assessments.

Topic Area: Methods